By Paul Graham, Prentice Hall, 1993 You can download entire book here. Even if you don’t know anything about LISP, there’s a lot to be absorbed from this book.

[p34] The trick is to realize that an imperative program is a functional program turned inside-out. To find the functional program implicit in our imperative one, we just turn it outside-in. Let’s try this technique on imp.

The first thing we notice is the creation of y and sqr in the initial let. This is a sign that bad things are to follow. Like eval at runtime, uninitialized variables are so rarely needed that they should generally be treated as a symptom of some illness in the program. Such variables are often used like pins which hold the program down and keep it from coiling into its natural shape.

However, we ignore them for the time being, and go straight to the end of the function. What occurs last in an imperative program occurs outermost in a functional one. So our first step is to grab the final call to list and begin stuffing the rest of the program inside it–like turning a shirt inside-out. We continue by applying the same transformation repeatedly, just as we would with the sleeves of the shirt, and in turn with their cuffs.